When irradiated nuclear reactor fuel elements are reprocessed, highly-active waste is obtained in the form of highly-active liquid concentrates of fission products. These liquid concentrates are solidified by appropriate vitrification processes. Glass-forming materials are added and the radioactive materials are melted to glass. The radioactive glass melt is placed in metal vessels made of special steel, so-called molds. After cooling, solidification and possibly quite long surface storage, it is intended that the glass-filled steel molds be taken to the ultimate storage location.
The vitrification of radioactive substances is advantageous for the reason that glass has a good resistance to leaching out. The chemical permanence of the storage package produced in this way is very high.
The melting of the glass-forming materials with the radioactive substances is usually carried out in a ceramic melting furnace directly heated by electricity. In the furnace the calcined fission product is continuously fused into the bath of glass melt. Metal vessels are then filled at intervals with the glass containing the radioactive substances.
Essentially three methods are known for filling the metal vessels from the glass melting furnace, namely: the bottom discharge system; the overflow system; and, the suction removal method.
In the suction removal method, prior to filling the steel mold, a partial vacuum is produced in the steel mold and the steel mold is then closed off so as to be vacuum tight. After a closed suction pipe, which is installed on the steel mold, is dipped into the molten glass bath from above, the partial vacuum in the steel mold causes the molten glass material to be drawn by suction into the closed steel mold after the closure on the suction pipe has melted. Therefore, in the technical language used in this art, the term "suction mold" has been adopted to denote a steel mold of this kind. In the descriptive passages which follow, the term "suction mold" is used for a metal container for receiving the molten glass material in accordance with the suction removal method.
In the suction removal method, a partial vacuum is established in the metal vessel and the vessel is sealed in a vacuum-tight manner. After a sealed suction tube mounted on the metal vessel dips into the glass melt from above and after the seal in the suction tube melts open, the partial vacuum in the metal vessel causes the glass melt to be drawn by suction into the closed metal vessel. The speed of filling the suction mold using the suction removal method is substantially higher than with the other mold filling methods.
The procedure of the suction removal method is disclosed in German published patent application DE-OS No. 29 27 795. In this process, the suction pipe which projects through a suction removal opening into the molten glass in the furnace is vacuum-tightly connected to the suction mold until the filling operation is concluded. Thereafter, the suction pipe is cut off and broken up and the broken pieces of the suction pipe are put into another empty container.
In relation to another procedure of the suction removal method disclosed in German Pat. No. 30 22 387, the suction pipe, which is fitted to the bottom of the suction mold to be filled, is pushed axially into the mold through an opening in the bottom thereof after the filling operation. The remaining opening in the suction mold is then closed by a container closure cover which is fitted into position thereon.
The essential part of the suction removal method, in terms of apparatus, namely the evacuated suction mold, is described in German published patent application DE-OS No. 31 04 366. The suction mold is provided with a suction pipe having a suction end which can be closed off by a fusible closure member. The closure member has a closable evacuation stub for evacuating the suction mold and the suction pipe. The evacuation stub is sealed in the closure part of the suction pipe and is arranged at least partially inside the suction pipe. The evacuation stub is made of a cold-weldable material which, for closing off the evacuation stub after the evacuation operation, is subjected to cold-press welding by mechanical deformation and thereby closed off. The cold welded metal connection of the evacuation stub provides a good vacuum-tight seal at the press-welded location.
After evacuation of the suction mold and prior to closing off the evacuation stub, a given residual gas pressure is set in the suction mold using dry gas.